Recent advances in ion chromatography have involved a technique referred to as gradient elution. This technique, in which the composition of the mobile phase eluent is varied over time, has been found to be very useful when analyzing samples containing a mixture of both weakly retained anions and strongly retained anions The most common method of gradient elution uses a two solvent gradient in which elution is begun using a weak solvent. This is followed by a continuous or stepwise increase of concentration of a second, stronger solvent until the solution consists entirely of the stronger solvent.
Unfortunately, gradient elution has been considered difficult or impossible to accomplish with liquid chromatography systems which utilize bulk property detectors such as those measuring conductivity or refractive index of chromatography column eluates. This is because the changes in composition of the mobile phases required during gradient evolution generally resulted in a change which was too large to be handled by the bulk property detectors. The relatively small deflections resulting from zones of analytes passing through the detector cell after their separation by gradient elution often were thought to remain undetected at the crude sensitivity setting imposed by the simultaneously occurring change in the bulk concentration of the eluent.
Recently, however, there have been reports of successful sample analysis via conductivity using the gradient elution method. See for example, Sunden, et al., Anal. Chem., 55, 2-4 (1983); Dasgupta, Anal. Chem., 56, 769-772 (1984); Tarter, Anal. Chem., 56, 1264-1268 (1984); and Shintani, et al., Anal. Chem., 59, 802-808 (1987). Additionally, attempts have been made using refractive index detection subsequent to gradient elution as reported by Berry, et al. in American Laboratory, 18, 57-66 (1986).
In each of the above references, the conductivity detection methods have been made compatible with gradient elution by employing external chemical devices, called suppressors, between the chromatographic separation column and the detector. While suppressors generally function to convert high conductivity signals to low level background readings, they can also be used to reduce the large change in conductivity which results from the conventional gradient elution techniques. Software methods are then required to subtract the baseline of blank gradients to improve the appearance of the obtained recordings. Additionally, the software methods enable an evaluation of peak heights and areas. In the absence of chemical suppressors, the computer aided baseline subtraction is insufficient to enable a reproducible evaluation of peak heights and peak areas when using conductivity detection in conjunction with conventional gradient elution techniques.